Regulated voltage (or regulated power) is essential for electronic circuitry. Related art provides two general approaches: a series voltage regulator, and a shunt voltage regulator. Each related art approach has advantages and disadvantages.
FIG. 1 illustrates a series voltage regulator 100 from related art. An unregulated voltage VDD is conditioned to a desired output voltage Vout by a series voltage regulator. The series voltage generator includes two major circuits: a pass circuit, and a pass feedback circuit. The pass feedback circuit attempts to maintain a constant desired output voltage Vout.
The pass circuit PC1 is configured to pass a pass current Ipass from VDD to output node Nout, wherein output node Nout has an output voltage of Vout. The pass feedback circuit PFC1 is configured to control the pass circuit as a function of Vout.
In one embodiment of a pass feedback circuit, Vout is reduced (or “voltage divided”) by a series of resistors R1 and R2, and the resulting voltage Vdiv feeds into error amplifier AMP6. For example, if R1 and R2 are equal, then Vdiv is half of Vout. Error amplifier AMP6 compares Vdiv to a reference voltage Vref, and outputs a feedback voltage or error voltage V6.
Specifically, the error amplifier AMP6 outputs an error voltage V6 proportional to the “error” between Vdiv and the reference Vref. This error voltage V6 adjusts (if necessary) the output voltage Vout. In FIG. 1, an excessive Vout causes a positive error voltage V6 and decreases the pass current Ipass, in turn decreasing the output voltage.
Specifically, in FIG. 1 the error voltage V6 from the pass feedback circuit PFC1 is tied to the gate of a PMOS power transistor MP6 in the pass circuit. If Vdiv exceeds Vref, then the error is positive (Vout is too high), the error voltage V6 is positive, and the positive voltage Vamp linked to the gate of PMOS transistor MP6 tends to open the normally closed transistor, thus reducing the pass current flow Ipass and reducing Vout.
Power transistor MP6 is known as a “pass” transistor, because output voltage Vout is controlled (at least partially) by passing current through the pass transistor towards output node Nout. Current flow Ipass is known as a “pass” current.
If Vout is reduced substantially, either by feedback effects or by a large load current (not shown), then Vdiv is reduced, the difference between Vdiv and Vref is reduced, error voltage V6 is reduced, and the voltage at the gate of PMOS transistor MP6 is reduced, thus tending to close the normally closed transistor MP6, increasing the current flow Ipass through MP6, and increasing Vout.
In light load (or no load) conditions, Vout is relatively high, Vdiv is relatively high, V6 is relatively high, the voltage at the gate of PMOS transistor MP6 is increased, and the current Ipass through MP6 is relatively low. Thus, the related art series regulator is efficient under light load conditions.
Further, the series voltage regulator provides high PSR (Power Supply Rejection) and good load and line regulation. However, the series voltage regulator has some drawbacks: the PSR has a narrow band, and the transient response is slow in light load conditions.
Shunt voltage regulators are discussed below. Shunt voltage regulators avoid some of these drawbacks of series voltage regulators, but also have their own drawbacks.
FIG. 2 illustrates a modified series voltage regulator 200 from related art. FIG. 2 is similar to FIG. 1, with the addition of an output capacitor Co to help damp out output voltage Vout fluctuations, and with unregulated voltage VDD serving as a power supply to AMP6. Alternatively, Vout may serve as a power supply to AMP6 (not shown).
FIG. 3 illustrates a shunt voltage regulator 300 from related art, comprising three major parts: a shunt circuit SC3, a shunt feedback circuit SFC3, and a constant current source Isource.
The shunt feedback circuit SFC3 is very similar to the above pass feedback circuit PFC1. The difference is that the error voltage V8 of error amplifier AMP8 is connected to a shunt circuit SC3 in shunt voltage regulator 300 (instead of to a pass circuit PC1 in the series voltage regulator 100).
Further, the shunt circuit SC3 works somewhat “backwards” from the pass circuit described above. In the shunt circuit SC3, a high error voltage V8 to transistor MN8 increases shunt current Ishunt, thus decreasing Vout. Also, shunt voltage regulator 300 requires the constant current source Isource in order to drive the voltage Vout.
Specifically, transistor MN8 in FIG. 3 is an NMOS transistor that is normally open (in contrast to the PMOS transistor in FIG. 1). Thus, in low load conditions Vout is initially relatively high, Vdiv is relatively high (greater than Vref), and a large error voltage V8 tends to close NMOS transistor MN8, allowing a large current (a shunt current Ishunt) to shunt through the transistor and towards ground, thus decreasing Vout.
Transistor MN8 is known as a “shunt” transistor, because output voltage Vout is controlled by shunting current through the shunt transistor away from node Nout and towards a ground.
Under high load conditions, Vout is initially relatively low, Vdiv is relatively low, V8 is relatively low, shunt transistor MN8 is relatively open (small shunt current Ishunt), and thus the output voltage Vout is driven higher due to Isource.
The shunt voltage regulator 300 gives relatively wideband power supply rejection (PSR), and relatively fast transient response. However, the shunt voltage regulator has a large current consumption at low loads because the current source Isource remains on at all times. At low load conditions, almost all of Isource is shunted as Ishunt through shunt transistor MN8.
FIG. 4 illustrates a shunt voltage regulator 400 with a capacitor Co from related art. FIG. 4 is very similar to FIG. 3, with the addition of an output capacitor Co to dampen fluctuations in Vout, and with Vout serving as a power supply to error amplifier AMP8. VDD may alternatively serve as a power supply to error amplifier AMP8 (not shown).
In FIG. 4, a small current I1 provides current to power error amplifier AMP8. Another small current I2 provides current for the voltage divider resistors R1 and R2 to generate Vdiv.
Other performance measures such as size and efficiency are also important in voltage regulators. The current source Isource may be created using various technologies including: bipolar transistors; zener diodes; and CMOS diodes. Of these options, only the CMOS diodes may be created with standard CMOS processes. However, these CMOS diodes must be sized for the maximum load current condition and operate under the maximum current condition at all load conditions (because these diodes form a constant current source). Also, the efficiency of a CMOS diode based constant current source is low, due to the almost 0.7V voltage drop across CMOS diodes. Further, the shunt transistor MN8 must be a very large size to shunt off virtually all of the current from the constant current source at the no load condition, because currents I2 (to resistor R1) and I1 (to the power supply input of error amplifier AMP8) are very small.
Compared to the series voltage regulator 100, the shunt voltage regulator 300 gives wideband power supply rejection (PSR) and fast transient response. However, the shunt regulator has high power consumption because the current source Isource is always on, and is shunted away through the shunt current Ishunt during periods of low load. This results in high power consumption during periods of low load.
Thus, there is a need for a hybrid voltage regulator that has the good qualities of the series regulator and of the shunt regulator, while avoiding the bad qualities of the series regulator and of the shunt regulator.